Lithium-ion capacitors (LICs) are promising next-generation electrochemical energy storage devices that combine the high energy density of lithium-ion batteries with the high power density of supercapacitors [1]. Thanks to these characteristics, LICs offer both rapid charge-discharge capability and high energy storage capacity, making them attractive for a variety of applications [2,3]. These include portable and wearable electronic devices that require lightweight and fast-charging power sources, electric vehicles that demand both high power and long driving range, large-scale energy storage systems for renewable energy integration, and backup power supplies for critical infrastructures. To meet the growing energy demands in these applications, electrode materials capable of adsorbing and desorbing a large number of ions are essential, as they can provide higher capacity [4,5]. Recently, biomass-derived activated carbon has garnered attention as an electrode material due to its abundance, low cost, and tunable porous structure favorable for ion adsorption and desorption [6]. In this study, oak mushroom (OM), an inexpensive and readily available biomass resource, was employed as a precursor for activated carbon production, offering both economic and environmental advantages. OM-derived activated carbon was employed as the cathode material, while lithium titanate (LTO) was used as the anode. The electrochemical performance of the LIC system was investigated through preliminary optimization of electrode thickness based on half-cell screening, followed by fullcell evaluation under optimized conditions. This study aims to elucidate the structural and electrochemical characteristics of OM-based LICs and to assess their potential as high-performance energy storage devices.

OM-derived activated carbon was prepared using oak mushroom as the carbon precursor. The oak mushroom biomass was first pyrolyzed at 550°C to obtain biochar. The resulting biochar was then thoroughly mixed with potassium hydroxide (KOH, ≥ 93%, Daejung) and subsequently activated by heat treatment at 800°C. After the activation process, the resulting material was washed, dried, and characterized in terms of its morphology and porosity. In this study, biochar refers to the carbon material obtained after the initial pyrolysis step, whereas activated carbon denotes the material produced after subsequent KOH activation and high-temperature treatment, which leads to enhanced porosity. For electrode fabrication, the OM cathode was prepared by mixing OM-derived activated carbon, Super-P, and SBR/CMC binder in a weight ratio of 8:1:1. To investigate the effect of electrode thickness, OM cathodes with coating thicknesses of approximately 60, 90, and 120 μm were fabricated and evaluated in half-cell configurations. Similarly, the LTO anode was prepared using commercial LTO, Super-P, and SBR/CMC binder in the same ratio (8:1:1), and electrodes with thicknesses of approximately 60, 90, and 120 μm were fabricated and tested in half-cells. The electrode thickness variation was conducted as a preliminary screening step to identify the optimal electrode configurations for the LIC system. Based on the half-cell results, the OM cathode with a coating thickness of approximately 60 μm, which exhibited the most balanced electrochemical performance, was selected for detailed discussion in the main manuscript. In contrast, the thickness-dependent electrochemical performance of the LTO anode is provided in the Supporting Information to demonstrate its stable behavior and to confirm that it does not limit the overall cell performance. Subsequently, the optimized OM cathode and LTO anode were coated onto aluminum foil and copper foil, respectively, and assembled into a full-cell (LTO//OM) using a coin-cell configuration with 1M LiPF₆ dissolved in EC/DMC (50:50 v/v, Sigma-Aldrich) as the electrolyte. The mass loading of the OM cathode and LTO anode was approximately 0.41mg cm^–2 and 0.88 mg cm^–2, respectively, corresponding to a cathode-to-anode mass ratio of 2.14. This electrode balance enables efficient ion and electron transport during charge–discharge processes and contributes to the stable electrochemical performance of the full-cell. The electrochemical performance of the LIC was systematically investigated through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), electrochemical impedance spectroscopy (EIS), and Ragone plot analysis under various conditions, including different scan rates, current values, and cycling tests. The capacitance from the cyclic voltammogram was calculated using Eq. (1):

Where C is capacitance (F, farad), I is applied current (A), t is discharge time (s), dV is the potential window (V). The degree of symmetry between charging and discharging was calculated using Eq. (2):

Where t_charge and t_discharge are the durations of the charge and discharge processes, respectively, measured for each current density.

The specific capacitance was calculated using Eq. (3):

Where C is specific capacitance (F g^–1), C_sp is specific capacity (mAh g^–1), and dv is the voltage range (V).

Fig. 1 (a) illustrates the preparation process, where OM was pyrolyzed at 550°C to obtain biochar, followed by chemical activation using potassium hydroxide (KOH) as the activating agent [7]. During the activation process, the reaction between carbon and KOH promotes etching and gasification, creating numerous micropores and enlarging pre-existing pores [8]. The resulting hierarchical porosity is essential for increasing the ion-accessible surface area [9]. Scanning electron microscopy (SEM) images in Fig. 1 (b) and (c) reveal a uniformly porous surface with pores approximately 10 μm, confirming that KOH activation successfully produced an interconnected porous framework [10,11]. Such a structure facilitates efficient ion adsorption/desorption and enhances charge transport, which are critical for improving LIC performance.

Fig. 2 (a) displays the nitrogen adsorption–desorption isotherms, which exhibit a Type H4 hysteresis loop, indicating the presence of well-developed nanoporosity. The corresponding pore size distribution (Fig. 2 (b)) reveals that the pore sizes are predominantly distributed in the range of 0.5-2.8 nm [12]. In addition, OM exhibits a significantly higher specific surface area of 2206.4 m² g^–1 and an average pore volume of 0.9146 cm³ g^–1 compared to the biochar, demonstrating the effective development of a hierarchical porous structure after KOH activation (Fig. S1 and Table S1). Such well-developed porous architecture facilitates enhanced electrolyte penetration and ion transport, which are beneficial for achieving high rate capability and capacitive performance. Based on the combined analysis of the surface morphology and pore structure, OM is confirmed to be an effective biomass precursor for producing activated carbon with a high surface area and well-developed nanoporosity [13,14]. These characteristics highlight the strong potential of OM-derived activated carbon as a porous carbon cathode material for supercapacitors and hybrid capacitors.

The X-ray diffraction (XRD) patterns in Fig. 3 exhibit a broad peak between 10° and 20°, which is characteristic of an amorphous carbon structure [15]. This broad feature reflects the disordered arrangement of graphene layers commonly observed in KOH-activated carbons. In addition, a weak diffraction peak appears at around 29°, which is attributed to residual potassium-containing species remaining from the KOH activation process [16]. Similar diffraction features have been reported for KOH-activated carbon materials, where trace amounts of potassium-related compounds may remain even after extensive washing. The relatively low intensity of this peak indicates that the residual species are present only in a minor amount and do not significantly affect the overall amorphous carbon structure. These results confirm that OM-derived activated carbon possesses a predominantly amorphous framework with minor inorganic residues from activation [17].

Fig. 4 (a) shows the XPS survey spectrum of OM-derived activated carbon, in which prominent C 1s and O 1s peaks confirm that carbon and oxygen are the major surface elements. The high-resolution C 1s spectrum (Fig. 4 (b)) was deconvoluted into four components located at 284.5, 286.7, 287.6, and 288.8 eV, corresponding to C-C/C=C (59.6%), C-O (18.0%), C=O (13.3%), and O-C=O (8.0%) bonds, respectively [18]. The dominant C-C/C=C contribution (59.6%) indicates a carbonaceous framework, while the substantial fraction of oxygen-containing functional groups (40.4%) suggests effective surface functionalization induced by KOH activation [19]. The oxygen functionalities are known to improve electrolyte wettability and facilitate ion adsorption/desorption processes. The O 1s spectrum (Fig. 4 (c)) was resolved into three peaks at 531.1, 532.9, and 535.5 eV, which are assigned to C=O (47.0%), C-O (41.0%), and O-C=O/-OH (12.0%) species, respectively [20]. The relative abundance of these oxygen functionalities further confirms the successful introduction of surface oxygen groups during the activation process, which is consistent with previous reports on similar KOH-activated biomass-derived carbon and beneficial for electrochemical charge storage behavior [21].

Fig. 5 (a) shows the CV curves measured at a scan rate of 10 mV s^–1 within various voltage windows. In all voltage ranges, the CV curves maintain a nearly rectangular shape, showing typical electrochemical behavior of an electric double-layer capacitor (EDLC) [22]. However, noticeable overvoltage is observed above 4.4 V, which is likely due to electrolyte decomposition at high voltages. Therefore, a potential window of 2.8–4.2 V was selected as the optimal range for evaluating the electrochemical performance. Fig. 5 (b) displays the CV curves obtained at various scan rates from 10 mV s^–1 to 200 mV s^–1. Even under high scan rate conditions, all CV curves maintain a rectangular shape, demonstrating excellent capacitive behavior. These results indicate that the OM can be a good candidate as an electrode material for hybrid capacitors. Fig. 5 (c) shows the capacitance retention calculated from the CV curves in Fig. 5 (b) according to Eq. (1). In this study, electrochemical performance descriptors are defined based on the capacitive-dominated charge storage behavior of the LIC system. Accordingly, all performance metrics are expressed in terms of specific capacitance (F g^–1), rather than specific capacity (mAh g^–1), unless otherwise stated. At a scan rate of 10 mV s^–1, the OM electrode exhibited a high specific capacitance of 360 F g^–1, which decreased to 75 F g^–1 (21% retention) at 200 mV s^–1. Herein, retention refers to the percentage of the specific capacitance (F g^–1) maintained at a given scan rate relative to the initial value obtained at 10 mV s^–1. In this EDLC-like LIC system, capacitance retention is used as a key performance descriptor to evaluate the rate capability of the electrode under increasing kinetic constraints. Although the capacitance decreased at high scan rates, the OM electrode still showed stable capacitive behavior, confirming its suitability as a cathode material for hybrid capacitors.

Fig. 6 (a) illustrates the charge and discharge profiles measured at current densities of 50, 100, 200, and 300 mA g^–1 within the voltage window of 2.8–4.2 V. At each current density, the charge and discharge curves exhibit the characteristic triangular shape without any redox plateaus, indicating that the electrochemical behavior of OM is governed by electric double-layer capacitance [23,24]. In addition, we quantified the degree of symmetry between charging and discharging by calculating the charge–discharge time matching ratio at each current density according to Eq. (2). Here, the matching ratio is defined as the ratio of discharge time to charge time obtained from galvanostatic charge-discharge curves and is used as a quantitative descriptor to evaluate the reversibility and symmetry of the charge-discharge process. A value of 100% indicates perfect time symmetry, whereas deviations from 100% indicate asymmetry arising from IR drop, kinetic limitation, or minor side reactions [25]. The measured matching ratios were 82.2%, 81.3%, 81.8% and 92.3% for 50, 100, 200, and 300 mA g^–1, respectively. These results show that charging and discharging times are comparable across the tested current range, indicating good electrochemical reversibility and efficient ion/electron transport in the porous OM electrode [26]. Fig. 6 (b) presents the cycling stability for 100 charge and discharge cycles. After 100 cycles, approximately 97% of the initial capacitance is retained, confirming excellent cycling stability, while the coulombic efficiency remains close to 100%, further demonstrating the high reversibility of the OM electrode. Fig. 6 (c) shows the specific capacitances at various current values. The specific capacitance was calculated using Eq. (3).

The rate capability of the OM cathode was evaluated by galvanostatic charge–discharge measurements at various current densities. At a current density of 50 mA g^–1, the OM electrode delivered a specific capacitance of approximately 108.4 F g^–1. As the current density increased to 300 mA g^–1, the capacitance gradually decreased to about 82.4 F g^–1, corresponding to a retention ratio of approximately 89%. Herein, the retention ratio refers to the percentage of specific capacitance maintained at a higher current density relative to that obtained at the initial current density. When the current density was subsequently reduced back to 50 mA g^–1, the specific capacitance recovered to around 107.8 F g^–1, corresponding to a recovery ratio of approximately 99.4%. In this study, the recovery ratio is defined as the percentage of specific capacitance recovered after high-rate operation when the current density is returned to its initial value. The high retention and near-complete recovery indicate excellent rate capability and structural robustness of the OM cathode. In addition, the LTO anode exhibits stable electrochemical performance with negligible degradation over the tested current range, as evidenced by the consistent rate capability and cycling stability observed in the half-cell measurements with different electrode thicknesses (60, 90, 120 μm) shown in Fig. S2.

Fig. 7 (a) shows the CV curves measured at various scan rates within the potential range of 1.0–4.2 V. All CV curves exhibit a quasi-rectangular shape with no clear redox peaks, indicating the typical electrochemical behavior of EDLC [27,28]. Fig. 7 (b) presents the Nyquist plots of the LIC full-cell measured before and after electrochemical operation, corresponding to the EIS spectra collected prior to and following cyclic voltammetry measurements. The EIS measurements were conducted in a potentiostatic mode at open-circuit voltage (OCV) using a single-sine excitation with an AC amplitude of 10 mV over a frequency range from 7 MHz to 100 mHz at room temperature. The impedance spectra consist of a depressed semicircle in the high-to-middle frequency region followed by an inclined line in the low-frequency region, which are characteristic features of lithium-ion capacitors. To facilitate interpretation, the corresponding Randles equivalent circuit is shown as an inset in Fig. 7 (b), where Rs represents the solution resistance, Rct denotes the charge-transfer resistance at the electrode/electrolyte interface, CPE accounts for non-ideal capacitive behavior, and Wo corresponds to the Warburg impedance associated with ion diffusion. The relatively small semicircle diameter indicates low interfacial resistance, suggesting efficient charge-transfer kinetics in the LIC system. After electrochemical operation, a slight enlargement of the high-frequency semicircle is observed, indicating a moderate increase in charge-transfer resistance at the electrode/electrolyte interface. Nevertheless, the semicircle remains relatively small, suggesting that fast interfacial charge-transfer kinetics are largely preserved. In addition, the slope of the low-frequency region shows no significant change, implying that ion diffusion within the porous electrode structure is not substantially affected. These results demonstrate that the LIC full-cell maintains stable interfacial and ion-transport properties during electrochemical operation [29]. Fig. 7 (c) illustrates the charge/discharge curves obtained at different current values. All curves exhibit the nearly linear and symmetric triangular shape characteristic of EDLCs, indicating that the charge storage mechanism is dominated by ion adsorption and desorption without faradaic reactions [30,31]. As the current values increase, the LIC full-cell maintains a well-defined triangular shape, and the absence of a noticeable IR drop increase indicates excellent reversibility and stable electrochemical performance [32,33]. Fig. 7 (d) shows the long-term cycling performance of the LIC full-cell. The cell delivers an initial specific capacitance of 139.5 F g^–1 and retains about 99% after 700 cycles, with coulombic efficiency remaining above 90%. These results confirm the excellent durability and stability of the LIC. Fig. 7 (e) presents the rate capability at current values from 50 to 2000 mA g^–1. The specific capacitance decreases from 92 to 68 F g^–1 with increasing current and recovers to 83 F g^–1 (90.2% of the initial value) when the current is reduced, indicating good reversibility. Although the Coulombic efficiency (~90%) is slightly lower than the ideal value for LICs, this can be attributed to the kinetic mismatch between the electrodes, suggesting that further optimization such as capacity balancing would be beneficial [34,35]. Nevertheless, these results highlight the high potential of biomass-recycled OM cathodes for practical LIC applications.

Fig. 8 presents the Ragone plot of the LIC full-cell. The Ragone plot is widely used to evaluate the performance of energy storage devices by comparing their energy and power densities. The energy density reflects the amount of energy that can be stored and delivered by the device, whereas the power density represents how rapidly the stored energy can be released [36]. Therefore, the Ragone plot provides a clear picture of the trade-off between energy storage capability and rate capability, which is critical for practical applications [37]. The Ragone-type energy and power densities of the LIC full-cell were evaluated based on the specific capacitance (F g^–1), reflecting the capacitive-dominated charge storage behavior of the OM-derived activated carbon cathode. All values were normalized by the cathode active mass. The specific capacitance (C, F g^–1) was calculated from the galvanostatic discharge curves according to Eq. (4).

Where I is the applied current (A), Δt is the discharge time (s), m_cathode is the mass of the cathode active material (g), and ΔV is the discharge voltage window (V), excluding the IR drop.

The energy density (E, Wh kg^–1) and power density (P, W kg^–1) were calculated using Eqs. (5) and (6), respectively.

Where Δt in Eq. (6) is expressed in hours.

The device delivers an energy density of approximately 113 Wh kg^–1 at a power density of 4600 W kg^–1 and maintains 86 Wh kg^–1 even at a high-power density of 14,000 W kg^–1. Compared with previously reported LICs, these values demonstrate that the fabricated LIC exhibits superior energy and power performance, highlighting its potential as a high-performance energy storage device. Overall, the performance of the as-fabricated LIC can be considered favorable compared with many previously reported LICs, including Hybrid capacitor [37], C@LTO//AC-20 [38], Quinone/ester-based LIC [39], 20Ni-hMO//AC LiHSC [40], 5-LVO//AC [41] LICs.

In this study, a lithium-ion capacitor (LIC) was successfully fabricated using oak mushroom–derived activated carbon (OM) as the cathode and LTO as the anode. The OM material exhibited a high specific surface area together with a well-defined hierarchical porous structure, which collectively enabled rapid ion diffusion and effective charge storage within the cathode matrix. These structural advantages contributed to stable electrochemical kinetics and facilitated efficient interaction between the electrolyte and the active surface sites. As a result, the full LIC cell delivered an energy density of 113 Wh kg^–1 and a power density of 14,000 W kg^–1, demonstrating a balanced combination of high energy and high power characteristics. In addition, the LIC full-cell maintained 99% capacitance retention after 700 cycles, confirming its remarkable long-term stability and the robustness of the OM cathode architecture. This performance highlights the structural benefits derived from biomass-derived material and supports the applicability of OM in practical LIC systems.